1. Field of the Invention:
This invention is related to journal air bearings and in particular to an air bearing having a slip ring construction that minimizes the problem of galling between the air bearing surfaces.
2. Prior Art:
Journal air bearings are bearings comprising a cylindrical rotor enclosed by a stator (or housing) in which the surface of the rotor is supported out of contact with the interior surface of the stator by a pressurized air film. The air film is the lubricant for the bearing and therefore a major advantage of this construction is the low frictional loss that is associated with the bearing. A second advantage is the minimal transmission of vibration from the stator to the rotor. Journal air bearings are therefore used in applications where elimination of vibration is very important such as to support the spindles of latches or grinders used to manufacture substrates for memory disks. Journal air bearings may be designed to support the rotor in both the radial and axial directions. Axial support is provided by a flange on each end of the rotor that is parallel and in close proximity to a stationary flange on each end of the stator. The interface between the rotor and stator flanges is an airbearing space, wherein pressurized air is supplied through a passage leading from a source of pressurized air, through the wall of the stator and opening into the airbearing region. Other constructions are also used to provide greater support depending on the application. These constructions include spherical interfaces which in one case is convex toward the axis of the bearing and in another case is concave toward the axis of the bearing. In another construction, the rotor is tapered.
The performance of any air bearing depends on the distribution of pressure and pressure gradients in the air bearing space. These distribution patterns are established by shallow grooves in the surface of the stator bounding the air bearing space and lead from the proximity to the entry of the air passage into the airbearing space to various areas of the interface.
For a detailed discussion of the prior art, reference is made to "Hydrostatic and Hybrid Bearing Design" by W. B. Rowe, published by Butterworth & Co., Univ. Press, and available in the Library of Congress, TJ1073.5.R69 1983.
A major problem with journal air bearings is their susceptibility to "crashes". A "crash" is the term applied to a situation where, for one of a number of reasons, oftentimes a speck of dirt, momentary contact between the airbearing surfaces occur leading to collapse of the air film, severe galling of the two surfaces moving in frictional contact with one another and sudden "freezing" of the rotor. When a crash occurs, the rotor must be removed from the stator, the surfaces lapped and polished, and the bearing reassembled. Usually the lapping must be performed by the manufacturer of the air bearing so that considerable time and effort is expended.
The traditional approach to avoiding crashes has been to pump very clean (filtered) air through the air bearing. Filters are normally designed to prevent only a fraction of particles larger than a given value from passing. Therefore the problem persists because it is virtually impossible to prevent particles from eventually passing through the filtering system into the airbearing space.
In order to minimize the damage due to the crashes, airbearing surfaces are made that are very hard. Surfaces that are used are "hard anodized" aluminum, and nitrided (case) hardened steel. The problem here is that preparation of these surfaces is costly and critical. Furthermore, when crashes occur involving these surfaces, the relapping that is necessary often "breaks through" the hardened surface layer and the parts must be retreated or discarded.
Another approach to minimize damage due to a crash has been to install a relatively expensive braking system that stops the rotation within a period of one revolution of the beginning of a crash. The latter remedy is obviously not effective by definition.